The Tropical Fruit Forum

Looking for some opinions on the ID of this Cherimoya please. The fruit itself is large and inedible. Seeds make excellent rootstock. I'm thinking a seedling of El bumpo maybe? just a guess, the tree itself is very old, maybe 40 to 50 yo, the house where it's planted was build in the 50's so could be older.
Lastly, the tree is a prolific self pollinator, with fruit on the tree all year. It's a big pity it's inedible, ie the fruit goes bad before it ripens, tried many fruits at many stages of ripeness.
The tree is located in Australia north of Sydney.

This fruit was picked green and early, today, still had more growing to do.

It's now the end of Fall here and winter is approaching fast. I thought I'd keep the summer going by adding some heat  to my newly completed poly house.  It (the polyhouse) measures 36ft x 18 x 12ft high at it's highest point. Minimum temps here in winter rarely go under 39°, but to try and match Mike T's climate in Cairns (Zone12), I had to pull a rabbit out of my hat. With the flue turned to it's lowest setting the minimum overnight inside temp so far got down to 68°.  Outside  50°. I have unlimited heavy hardwood here mainly a eucalypt known as ironbark, an excellent slow burning timber. I'll only need one large piece a night if it works this well on low. I will eventually run the flue along the long length of the polyhouse before exiting the other end to make it more efficient. The heat will carry and dissipate more efficiently with the much longer flue on the inside.